1. Field of the Invention
The present invention relates to a display apparatus, and more particularly, the present invention relates to an organic electroluminescence (hereinafter, abbreviated to EL) display apparatus.
2. Description of the Related Art
Display apparatuses that use emission of light of organic EL elements, especially, active-matrix organic EL display apparatuses that are controlled using a thin-film transistor (hereinafter, abbreviated to TFT) have advantages as compared to liquid crystal displays, for example, the organic EL display apparatuses can vividly display images and the apparatuses can be thinly made, and thereby have attracted attention.
However, there is a problem that the luminance of the organic EL elements decreases after long-time emission of light. FIG. 2 illustrates change of the luminance of organic EL elements to which a certain amount of electric current is applied to emit light continuously. The vertical axis indicates the rate of decrease from initial luminance, and the horizontal axis indicates elapsed time (unit: time). In the figure, the three curves indicate initial luminance of 100 cd/m2, 300 cd/m2 and 500 cd/m2 respectively. The magnitude of the electric current flowing through the elements is great in this order. As illustrated in the figure, as the initial luminance is higher, in other words, as the magnitude of the electric current is greater, the degradation of the luminance is faster.
The reason for the degradation of the luminance has not yet been fully understood. Given the fact that the light-emitting face has not reduced, it is thought that the degradation is not due to an external factor, but due to change of the inside of the organic EL elements generated along with the emission of light. The speed of the degradation of the luminance depends on the magnitude of the electric current per unit area of the light-emitting face, in other words, the current density, so that if the current density is high, the degradation of the luminance proceeds fast.
In a matrix display apparatus, when a certain white pattern is displayed with a black background, and the display is repeatedly performed for long periods of time, organic EL elements in the black part are in a light-off state, so that the luminous efficiency of the organic EL elements in the black part does not degrade. On the contrary, the luminous efficiency of the organic EL elements in the white pattern part degrades. As a result, when another image is displayed, a luminance difference between the area where the white pattern has been displayed and the area where the black background has been displayed is generated. When the luminance difference becomes visible to human eyes, the luminance difference is recognized as burn-in of the pattern. The burn-in is a phenomenon that a part of a previous screen can be seen as an image lag when the screen is switched. The burn-in significantly deteriorates the image quality.
In digital cameras, information such as an operation mode of the camera is displayed on a liquid crystal or an organic EL monitor display. The information is displayed at the same position on the monitor screen as a certain pattern such as an icon, thereby burn-in is generated.
Japanese Patent Application Laid-Open No. 2005-037847 discusses means for reducing burn-in in digital cameras. In the method, when a white icon is displayed, a white balance is changed, and within a range the user can recognize the icon image as a white image, the luminance of a most deteriorating color is decreased to the luminance lower than that of the other colors.
Monitor displays of portable apparatuses such as digital cameras that are often used outdoors are required to maintain sufficient viewability even under bright external light. However, in order to prevent burn-in, if the luminance of the pattern itself for displaying characters or an icon is lowered, the displayed pattern is hard to see because, for example, in the outdoors where the intensity of external light is high, the ratio of reflected light increases.